As families try to find safe-ish ways of holding holiday gatherings over the next few months, air purifiers have emerged as a possible method to mitigate the risk of spreading of COVID-19 among loved ones. Ventilation, we already know, is a crucial layer in the swiss cheese model for coronavirus, a public health graphic that illustrates the fight against respiratory viruses as one that requires multiple interventions working together. Given the way the virus is believed to linger in the air, floating around in invisible aerosols expelled by infected people, ventilating the air can meaningfully reduce the level of virus in the air.
Most infectious disease experts recommend simply opening a window to achieve reasonable ventilation if someone in your household has had known exposure to or is infected with COVID-19. But most places across the country are starting to get pretty cold; opening all the windows is no longer an option. And so, air purifiers.
To answer the question of who needs an air purifier, and whether a little machine you can plug in to your wall is a golden ticket to having the whole family over this season (hint: it’s not), VICE spoke with three industrial hygienists, whose entire jobs revolve around keeping the air inside our homes, hospitals, and office buildings safe.
What do you mean by “air purifier”?
Though they’re often sold as “air purifiers,” the machines in question are more accurately called air cleaners. Richard Corsi, an industrial hygienist and dean of the Maseeh College of Engineering and Computer Science at Portland State University, explained to VICE that air purifier falsely implies that these devices completely purify the air, when what they really do is clean a lot of the junk out of it. By “junk,” we mean: allergens, dandruff, dust and particles from construction sites and wildfires, and, if equipped with a HEPA filter, COVID-19 aerosols.
On a large scale, air cleaners are used in surgical theaters and hospitals to keep the air as clear of viruses and other contaminants as possible. But you can also buy smaller versions to keep at home, like a lot of people who live in wildfire-affected areas do.
Depending on the size of the room it’s in and the size of the machine itself, an air cleaner works by exchanging the full volume of air in a room or building a certain number of times per hour (this is known in the particular context as the “exchange rate”). One of the main takeaways is that industrial hygienists and scientists believe the ideal exchange rate for COVID-19 is somewhere between six and 10 times per hour, according to David Krause, a certified industrial hygienist and toxicologist in Florida.
While the exchange rate won’t be listed on device packaging, you can calculate it using the listed clear air delivery rate, or CADR, which should be in the product description or on the box. We’ll get more into the simple math required to do this below.
But some ventilation is better than none, so even if you can’t achieve that—or simply don’t want to buy the handful of devices that may be required to pull this off in your home—a single, small device (or, again, just opening a window) is going to provide some degree of protection.
Sweet. So should I get one?
It can’t hurt. But consider the value it would provide, based on your current living situation. A good air purifier is expensive, likely somewhere around $200–300. If you have been behaving well, following the rules of masking, social distancing, and minimizing your movement outside of your home, an air purifier may not be worth the cost.
Peter Raynor, professor of environmental health science at the University of Minnesota, said the best use of an air cleaner during COVID-19 would be if someone in your household is infected. “Reducing the level of virus in the air so that other people in the home are less likely to be infected is a really appropriate use of the room air cleaner,” Raynor told VICE.
Raynor clarified that he wouldn’t qualify an air cleaner as “essential,” in the way that masks and social distancing are. That mostly comes down to cost. If you’re already following the rules of this pandemic, air cleaners are a nice-to-have, rather than a thing-you-should-absolutely-do-to-prevent-mass-infection.
If you decide you need an air cleaner, make sure you get one that has a true HEPA filter. Developed during the Manhattan Project to capture radionuclides, HEPA filters are what you need to capture the tiny particles that contain COVID-19 and other viruses.
Yeah, but I want to have people over. If I get an air purifier, can I do that?
All three industrial hygienists were very clear that it’s not advisable to have family, or any people who don’t live in your household, come over for the holidays or any other occasion at this point in the pandemic. They aren’t alone in this recommendation. The CDC has warned that small, family gatherings are a common venue for spreading COVID-19, and with the holidays coming up, this should serve as a very clear warning.
But. If you, or your boomer parents, or your anti-masker uncle, absolutely insist on hosting a family gathering this season, having a good air cleaner plugged in certainly helps.
Great! How do I best use an air purifier?
This is where math comes in. Harvard and the University of Colorado Boulder have a Portable Air Cleaner Calculator you can use to figure out the size of air purifier you should buy for your home. To use it, you need to know the square footage of the room (or rooms) you want to be cleaned, or in other words, measure the square footage of the areas where people will be hanging out in your home.
Corsi said you’ll likely want something with a CADR around 300. (Below is a screenshot from an Amazon description of a Levoit air purifier, showing what the CADR looks like.) You’ll use this figure to calculate the device’s exchange rate, too. Let’s say the CADR is 300. To find out the exchange rate, take 300 and divide it by the volume of the room you’re trying to clean (meaning length x width x ceiling height). Multiply the resulting number times 60 (as in, 60 minutes in an hour) and you’ll get the exchange rate. While between six and 10 is ideal for COVID-19, anything above three is pretty good, Corsi said.
He also added that the CADR listed on all air cleaners only applies to when the device is running on its highest setting. Both Corsi and Krause recommended running your air purifier on its highest setting, especially if you have people who don’t live in your household coming over. It’ll be louder, but what’s a little noise in the interest of preventing the spread of a fatal virus?
Krause also added that you should keep your filter in the middle of the room, unobstructed by any furniture, and, if you can, prop it up on a chair or table so that it’s in “breathing space,” or at the same level as everyone’s face and aerosol-spewing mouths/noses. If you have two purifiers in a larger room, put them on opposite sides of the room and prop them up.
Do I have to close my windows if I’m running the air purifier?
Actually no; this is a somewhat common misconception. Corsi said that, rather than rendering the purifier useless, opening a window (weather permitting) enhances the amount of air filtration taking place. You’re just introducing even more new air to the space, essentially giving the air purifier a little organic assist.
So if I have a properly sized air purifier and keep all my windows open, I’m good to go mask-free around grandma?
No! Once again, air cleaners are but a single slice in the big hunk of swiss cheese that is preventing spread of COVID-19. An air purifier is a good addition, for some people, to masking, social distancing, and limiting contact with other people.